Copper indium gallium diselenide (CIGS) is a commonly used absorber layer in thin film solar cells. CIGS thin film solar cells have achieved excellent conversion efficiency (>20%) in laboratory environments. Typical voltage-current characteristics, known as the IV curve, of a diode without and with illumination demonstrates that an applied potential is in the forward bias direction. Such an IV curve shows the turn-on and the buildup of the forward bias current in a diode. Without illumination, no current flows through the diode unless there is external potential applied. With incident sunlight, the IV curve shifts up and indicates that there is external current flow from the solar cell to a passive load. Short circuit current, Isc, flows with zero external resistance (V=0) and is the maximum current delivered by the solar cell at any illumination level. Similarly, the open circuit voltage, Voc, is the potential that develops across the terminals of the solar cell when the external load resistance is very large. The power delivered to the load is of course zero at both extremes and reaches a maximum (Pmax) at a finite load resistance value. Pmax is typically shown as the area of a shaded rectangle. A commonly used number that characterizes the solar cell is the fill factor, FF, which is defined as the ratio of Pmax to the area of the rectangle formed by Voc and Isc.
The efficiency of a solar cell is the ratio of the electrical power it delivers to the load, to the optical power incident on the cell. Maximum efficiency is when power delivered to the load is Pmax Incident optical power is normally specified as the solar power on the surface of the earth which is approximately 1 mW/mm 2 Techniques to improve efficiency can cause other issues that tend to reduce the effective usefulness of the solar cell.